Is the XMB Bootable yet????

But would it be possible to boot the XMB in RPCS3?

It would be only possible if devs would wanna emulate stuff that doesn't even needed for games, so my guess is short answer is no.

If you want XMB, check out ex-rpcs3 dev's (at least i dont see him controbute since long time ago) emulator, from the looks of it it will emulate xmb https://github.com/AlexAltea/nucleus , have fun waiting 10-15 years until it becomes bootable :p